How to align or mirror half armature
In edit mode, you can access to the "symmetrize" function:


Select the left part of the character
The use the menu "Armature">"Symmetrize"


It will work if the bones are correctly named (".L" or ".Left" suffix for the left part, ".R" or ".Right" for the right part).



Note that it is working also if the bones are not already created:



You can also use the "X axis mirror" option:



Note: all these functions work well if the armature is along X with no rotation. So if you have rotations, apply them before CrtlA and choose rotation)